web3 is the shell user of the site while mysite-test is a alias entry with the same uid for web3 hat is used for logging in, so if you create a shell user to access that site, the shell of the main shell user of the site has to be changed too.

The shell also depends if jailkit was used for the site. You might want to read the jailkit and ispconfig sources if you like to know in detail whan which shell is set and required for the system to work properly.